1. Field of the Invention
This invention relates to the purification of crystalline solids crystallized from a crude, concentrated aqueous feed solution. More particularly, an aqueous crystal slurry withdrawn from a continuous crystallizer is treated in a double hydroclone procedure to recover crystal solids that are relatively low in impurities.
2. Description of the Prior Art
Most soda ash in the United States is refined from trona ore, a crude form of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O). The soda ash (Na.sub.2 CO.sub.3) is obtained by crystallization of a sodium carbonate species from crude concentrated aqueous feed solution derived from solubilization of the ore.
In one soda ash process, an aqueous slurry of sodium carbonate monohydrate is recovered from a crude, concentrated aqueous sodium carbonate solution by evaporative removal of water in a continuous crystallization operation. Impurities (like sodium chloride and sodium sulfate) present in the crude sodium carbonate feed solution become concentrated in the liquor of the crystallizer slurry and contaminate the recovered crystallizer slurry and contaminate the recovered crystalline sodium carbonate.
The crystalline sodium carbonate recovered from the crystal slurry may be water-washed to remove impurities from associated crystallizer liquor, but such washing results in loss of alkali values from dissolution of the solid sodium carbonate into the wash liquor. Such wash liquors usually contain dilute concentrations of sodium carbonate that are not directly recoverable because of the presence of impurities. Recycle of these dilute liquors to the crystallizer is undesirable because such wash liquors reduce crystallizer efficiency owing to their diluent effect and the consequent need to remove additional water.
The process of this invention provides a simple but ingenious way of purifying a crystalline species, particularly sodium carbonate, recovered from an aqueous crystal slurry, without resorting to water washing procedures or the introduction of other aqueous streams that reduce the overall efficiency of the continuous crystallization operation.